Help! I need to undue a single flow action which changed the data value for all my leads' email address to a single one

I would get in touch with Support and log a ticket ASAP.  Perhaps there's something they can do to reverse it.

Otherwise, if Marketo is hooked into a CRM system, you could always sync email addresses back over from your CRM or export/import a list and upload it that way.

In the worst case scenario, the old email address is still stored in the Activity Log and can be viewed there - so there's still a record of it in the System.  That's not a feasible way to update back in bulk, however.

Ouch! You have my sympathies. 

Laura's suggestion is a good one although you may find all the addresses in the CRM are updated as well to your own. 

In which case you may want to see if there is a backup of your CRM data you can use to restore the email addresses. 

 

You may also be able to get a report in your CRM that shows contact/lead history. I am not sure, but that may show you the old value of the email address before you changed it. 

yikes. this happened to me on a small scale recently (100 leads or so) where the email addresses were erased...the only option on the marketo side was to go into the activity logs and re-enter the email addresses manually, as Laura mentioned.  Maybe there's an audit trail or something you can revert from on the CRM side, though I'm not sure what CRM you are using?  Good luck with this